1. Field of the Invention
The invention relates generally to the field of chemical treatment systems for use with hydrocarbon producing wells. More specifically, the invention relates to chemical treatment systems which inject “pre flushed” or diluted treatment chemicals into the well.
2. Background Art
In wellbores drilled through the Earth and then used for production of hydrocarbons, a pipe or casing is disposed in the wellbore from the Earth's surface to the bottom of the well. The casing serves to hydraulically isolate the various Earth formations penetrated by the wellbore and to provide the wellbore with a degree of mechanical stability. Typically a tubing string, which is a pipe of considerably lesser diameter than the casing, is positioned within the well casing. The purpose of the tubing string is to enable produced fluids to move to the Earth's surface at greater velocity than would be possible within the casing. The hydrocarbons, and in many cases a considerable amount of connate water, enter the tubing through perforations located at the lower end of the casing, travel through the tubing, to a wellhead at the Earth's surface. In some wells, where the natural fluid pressure in the Earth's subsurface is not sufficient to lift the produced fluids to the Earth's surface, the fluids are pumped to the surface with a “sucker rod” pump or with a downhole electrical submersible pump.
At the Earth's surface, various production equipment directs the produced fluids to holding tanks and/or to a pipeline. The production equipment typically comprises tubing, valves, piping, and other components. The produced fluids typically contain numerous compounds which adversely affect the production equipment. For example, paraffins and water/oil emulsions can coat well production equipment and can eventually plug off the tubing and/or plug the perforations in the casing. In addition, chemical reactions between the produced fluids and metallic equipment can cause scale to be formed on the well production equipment, and some compounds in the produced fluids can corrode the well production equipment.
Various techniques are known in the art to treat these well conditions to extend the useful life of the well production equipment, tubing and casing. In wells susceptible to paraffin build-up, for example, “treater trucks” or “hot oil trucks” are regularly dispatched to pump heated oil and/or heated water into the well. The heated oil and/or water is pumped into the well through the annular space between the tubing and the casing, travels down through the annulus to melt the paraffin deposits in the well production equipment, and the returns to the surface through the tubing. In wells susceptible to corrosion and scale problems, high pressure injection treater trucks pump batches of chemicals into the well to chemically remove the scale, and to inhibit the causes of corrosion. All of these techniques require regular maintenance services which are costly and which do not continuously treat the well. Treater truck or batch treatment of wells is less efficient than continuous treatments because more chemicals are typically injected in batch treatment operations.
To avoid inefficiencies associated with treater truck maintenance of hydrocarbon producing wells, it is known in the art to use mechanical pumps to inject chemicals into a well. Typically, mechanical pumps are supplied from a storage tank which holds the chemicals. The mechanical pumps and storage tanks are located adjacent the well for several reasons, such as for reducing the length of power cable or piping that connects the pump to a power source such as electricity or natural gas. The tanks are located above the pump and the chemical is gravity fed to the intake port of the pump. The tanks include a vent at the upper end of the tank to prevent a vacuum from developing in the tank as the pump draws chemical from the tank. In addition, the vent releases excess pressure within the tank caused by thermal expansion of the chemical. Such thermal expansion can cause the chemical vapors to be released into the environment through the vent. In addition, thermal expansion can cause the chemical to be ejected through the vent or through the sight glass used to indicate the chemical level in the tank. In either event, chemical vapors or the chemical fluids are released in an uncontrolled manner and can pose a hazard to personnel and to the environment.
The mechanical pumps used in typical chemical injection systems are powered by electricity or gas and include numerous moving components. It is customary to inspect these pumps on a regular basis, sometimes daily, to verify the operability of the pumps. Because the chemical is gravity fed to the intake of the chemical pump, sediment in the tank or the chemical settles toward the pump intake and can interfere with the operation of the pump In addition, the presence of an air bubble in the intake line may impede the operation of the pump because of a vapor lock. In such event, maintenance personnel routinely open a bleeder valve on the pump and release chemical from the pump until the air bubble has been cleared. This practice is undesirable because it releases chemical into the environment. An additional consideration with respect to venting a chemical pump system to the atmosphere is introduction of moisture into the chemical system, which may condense and foul the chemical.
One device known in the art for providing controllable, continuous chemical treatment for well production equipment is disclosed in U.S. Pat. No. 5,209,300 issued to Ayres. An apparatus and method described in the Ayres '300 patent include a vessel which holds the chemical and a pressurized gas which exerts a pressure on the chemical. A pressure regulator and a valve selectively control the injection of the chemical into the well as the pressurized gas urges the chemical out of the vessel. The pressurized gas drives the chemical through the regulator, valve, and into the well without venting the chemical or pressurized gas into the ambient environment. The apparatus described in the Ayres '300 patent is adapted to inject chemicals into the well in essentially undiluted form. As will be explained below, in certain cases it is desirable to pump undiluted chemicals into a wellbore and follow such injection with a water flush.
So called “batch treatment” is among the more widely used methods used for downhole treatment of sucker rod pumped wells. A treater truck is dispatched to the well. The chemical is placed into the annulus in undiluted form, and is followed by an “overflush” of water to assure proper treatment of the well, because sucker rod pumped wells are susceptible to “pump off”, whereby the pump is operated at such a rate as to essentially remove all the fluid from the well down to the level of the pump In such cases, there is substantially no fluid to dilute and disperse the treating chemical, making the treatment less effective. The overflush is intended to provide sufficient dispersing fluid for the treatment chemical so that the chemical can reach the bottom of the well. The chemical would otherwise need to travel the entire distance from the surface in undiluted form, often against upward flow of gas in the well. Typically, the chemical can dry out inside the well before ever reaching the bottom when injected in undiluted form. The overflush water, however, is typically taken from storage tanks located near the well for storing, and subsequent environmentally safe disposal, of connate water that is produced from the well along with oil and gas. The water in the tanks is often contaminated with high levels of oxygen and sulfate reducing bacteria. When reinjected into a well, such cross-inoculation of contaminates lessens the effectiveness of typical corrosion inhibitor chemicals, among other problems. The truck batch treatment process in effect provides a recirculation of the corrosive materials through the entire well, and any other wells sharing the same surface production equipment, on a periodic basis. Some method is needed to break this cycle so that the fluid used to protect the system is not part of the problem.